Optical fiber has found widespread application as a long haul transmission medium for voice and data transmission. For instance, substantially all of the newly installed capacity in the long haul portion of the public switched telephone network in the U.S. is optical fiber-based.
Although optical fiber at present is not widely used in the feeder and distribution portion of multi-user networks, e.g., the telephone network, extension of the use of optical fiber into this portion of networks is desirable and is expected to occur within the near future, resulting ultimately in all-optical communication systems.
Since typically the equipment and labor costs for connecting a subscriber, or a group of subscribers, to a central office or other switching station is a major portion of the total cost of a communication system, the ability to provide such connection at relatively low cost is of utmost significance. It is generally true that a large portion (possibly as high as 70-80%) of the media costs (cables, connections, fanouts, enclosures, and pedestals) of a conventional lightguide distribution system is cable cost. Thus, there exists a strong incentive to reduce the amount of cable in a system.
Various architectures for lightguide distribution systems are known. See, for instance, H. Kobrinski, Proceedings of the SPIE, Vol. 568, pp. 42-49, San Diego, August 19, 1985, in which star and ring network configurations are discussed in the context of multiple-access and broadcasting optical fiber networks using dense wavelength division multiplexing (WDM).
Star-type networks are also disclosed in International patent application PCT/GB86/00018 (International Publication No. WO 86/04200). As will be readily appreciated, a star-configured network is generally not very economical with regard to the required length of transmission cable. On the other hand, ring networks may pose, inter alia, access and collision avoidance problems, and frequently do not match well the geometry of residential subscriber networks. Network architectures are also discussed, for instance, in C. A. Brackett, Proceedings, International Communications Conference, Toronto 1986, page 1730; and M. S. Goodman et al., ibid, page 931. Dense channel packing WDM distribution systems of the broadcasting type using coherent detection have been proposed. By a "dense channel packing" WDM system we mean herein a system having at least about 20, frequently more than 50, remote stations, with a typical spacing between wavelengths being 15 nm or less. Components potentially useful in such systems are discussed in T. B. Meriem, Telecommunications Journal, Vol. 52(7), page 408 (1985). Such systems generally require a widely (e.g., more than about 10 nm) tunable local oscillator at each remote station (subscriber). Such oscillators (lasers) are currently not commercially available and can be expected to be relatively costly once they do become available. Furthermore, the use of a widely tunable laser on the subscriber premises can be expected to pose control and stabilization problems.
Prior art dense packing WDM lightguide distribution system architectures thus typically would be relatively costly to implement, since they use relatively large amounts of optical fiber and/or require the use of widely tunable lasers on the customer premises.
Furthermore, WDM architectures which are satisfactory for a small number of wavelengths will frequently not be satisfactory for dense channel packing WDM (e.g., if the number of wavelengths N is greater than about 50). In any realistic system the multiplexing loss and the demultiplexing loss typically should not substantially exceed about 10 dB each. The use of some simple broadband couplers (e.g., balanced Y couplers) to accomplish the multiplexing and demultiplexing can severely limit the number of wavelengths, since each such coupler may introduce a 3 dB loss. See, D. H. McMahon, Journal of the Optical Society of America, Vol. 65(12), pp. 1479-1482 (1975), especially Example 3. On the other hand, narrow band couplers do not necessarily cause such large losses, and might conceivably have an average loss of only about 0.2 dB per coupler, due to unavoidable imperfections, finite passband width, and the like. A simple system that uses only narrow band couplers thus could perhaps accommodate up to about 50 different wavelengths.
In view of the potential importance of end-to-end optical communications, it would be highly desirable to have available a distribution system architecture that requires a relatively small quantity of optical fiber, that does not require the presence of widely tunable laser local oscillators at the remote stations, and that can accommodate a relatively large number of remote stations (subscribers). This application discloses such a system.